Heat resisting coating composition and method of applying it to the basic refractory surface of a furnace



United States Patent HEAT RESISTING COATING COMPOSITION AND METHOD OF APPLYING IT TO THE BASIC RE- FRACTORY SURFACE OF A FURNACE Raymond Jules Demaison, Bronx, N.Y., and Lewis J.

Dreyling, Milltown, N.J., assignors to Quigley Compauy, Inc., a corporation of. New York No Drawing. Filed Feb. 12, 1959, Ser. No. 792,701

7 Claims. (Cl. 106-59) This invention relates to roof maintenance for certain important kinds of industrial furnaces. In former Patent No. 2,809,126 there is disclosed a procedure which is applicable to various types of refractory furnace walls, whether they be made of silica, 'magnesite, chrome-magnesite or chrome. That process and composition has gone into very extensive use.

In accordance with the present invention, however, it has been found that where the furnace roof or wall is constructed of magnesia, chrome magnesia or chrome, still more effective results can be achieved by a new composition and process herein disclosed.

As brought out in said former patent, the furnaces for which this invention is particularly applicable, are the open hearth steel smelting furnaces where high temperatures of from 2800 to 3000 F. are maintained for protracted periods, and reverbatory furnaces for the smelting of copper where the temperatures range up to 2800 F.

In such cases, the high temperatures and conditions within the furnace cause the inner surface of the roof or wall to deteriorate. Even with the best maintenance formerly available, frequent suspensions and shut downs of the furnace are required, for patching, repair of the lining or reconstruction of the furnace wall.

In the above-mentioned patent, there is disclosed a process of roof maintenance by spraying upon the inner surfaces of the furnace while the furnace is in operation a special coating composition which, as it is sprayed on the hot surfaces, first softens enough to become adherent to said surfaces, and then hardens into a refractory cohesive lining which is sufiiciently yielding that it will not crack off in response to temperature changes, or deterio rate under atmosphere conditions within the furnace.

The invention of the prior patent can be successfully applied to refractory walls, whether they be silica, chrome, chrome magnesia or magnesia.

It has now been discovered that when dealing with walls of a basic character such as magnesia, chrome magnesia or chrome, still better results can be produced by using a new composition which reacts upon the basic furnace walls in a manner to produce a still more durable lining and which permits such furnace wall repairs to be made at much longer intervals. The amount of patching required is considerably reduced, and the length of time the furnace may be operated continuously without rebuilding is materially extended. if desired, to operate the furnace at higher temperatures without undue deterioration of the walls.

Heretofore, while magnesia has been used as a furnace wall material because of its very high melting point, it has It is also possible, i

not been found practical to use it as sprayable repair 1 ice with a chrome ore material, such as is disclosed in the former patent. Thus, from one point of view, in the new composition we may regard the material of our former patent as a matrix carrying a limited magnesia content and bonding it to the basic furnace Wall until, under the protracted heat of the furnace, some new crystallization occurs which is still more refractory than the former composition, and which is more resistant to deterioration and more adherent to the furnace walls.

The composition of this invention, in its most effective forms, comprises:

Chrome ore containing a predominnant amount of chromite 82*92 parts. Free iron oxide present in the chrome ore or added to the composition fluxing agent) 1-6 parts. Free magnesia (dead burned or fused) 1040 parts. Binding agent -n 6-12 parts. Dispersing agent O.25'5 parts of the binding agent.

Suspension agent 1-5 parts.

The invention also comprises the mixing of such material with water and spraying it in slurry form upon the basic furnace wall or roof while the furnace is in full heat, and' applying successive thin layers of the composition to the hot surface, whereby each layer is subjected to the full heat of the furnace, permitting the interaction between the furnace material and the sprayed material to occur step by step, the ingredients of each successive layer being blended and entering into union with the surface to which it adheres until a hard refractory surface is formed, thereby permitting the intercrystalline relation betwen the roof or wall material and the coating to take place without unduly affecting the roof temperature. This manner of applying the coating material in very thin layers insures the maximum adhesion of the coating to the furnace wall, because it permits the desired reaction to occur as the operation proceeds.

Referring now again to the composition, the chrome ore will usually be predominantly an iron chrome spinel, but these ores customarily include other oxides, such as magnesium oxide and aluminum oxide. Such ores frequently also include free oxide of iron, which if present in the proper proportions may be utilized to constitute the fluxing agent. If the quantity is insufficient for this purpose, additional iron oxide may be added. In general, the MgO present in the ore is not in a form to serve as a substitute for the additional free magnesia required by this new invention.

Due to the presence of the additional free magnesia, the quantity of free iron oxide may be less than in our former patent, and may be as low as 1%.

The dead burned or fused magnesia of the new formula is M gO in its most refractory form, normally having a fusing temperature of substantially 4000" F. The magnesia and the chromite are suspended in a sort of plastic mass by the iron oxide as it softens at furnace temperatures and holds the chrome material and the magnesia upon the furnace wall. There, under the influence of the furnace heat, the chrome and the free magnesia of the composition and the material of the furnace wall enter into some form of recrystallization, intimately uniting the sprayed mate-rial to the basic furnacevwall, and blending them together to form a very refractory surface intimately connected to the basic furnace Wall, but having a degree of yielding which enables it to retain its form without cracking or spalling with changes in furnace temperature. 'The free MgO added to the chrome helps to unite the lining to the surface of the wall, and at the same time increases the refractory characteristic of the lining itself.

acaaaae The chemistry involved is not wholly clear, but it is known that these oxides of the formula, when under high temperatures, form either mixed oxides, or form solutions of one oxide in another, and that these mixed oxides are very refractory. A possible explanation may include some recrystallization or crystal solution between the chrome content, the magnesia and the oxides of the furnace wall, in combination with the iron oxide. The crystalline formation that takes place at these high temperatures cannot be determined with accuracy, but can only be inferred from the extraordinary results which follow the addition of the stated quantity of MgO to the predominantly chromite mixture.

The binding agent, as in the former patent, is one or more of conventionally known binders, such as the sodium silicates. Its soda to silicate ratio may be about 1 to 2, but it is preferably 1 to 1.9, that is Na O:1.9SiO but other binders, such as Na O:2SiO and Na O:3.22SiO are also available.

A dispersing agent, also as in the former patent, is preferably added, and it may be (a) A known conventional agent, or (b) Metal salts of amyl sulphonic acid, or (c) Sulphonated petroleum products.

This dispersing agent improves the action of the binding element, cooperating to render the binder more adhesive and cohesive.

A suspension agent is also added, such as colloidal clay, or an organic suspen'der, such as carbox'y-methyl cellulose.

The proportions of the refractory ingredients of the coating composition can be adjusted or varied dependent on the temperature of the furnace and the other conditions of its use.

The present invention includes the discovery that for the best results the correct proportions of components in the chemical composition of the refractory material or cement should be adjusted or caused to vary, depending upon certain factors, including, first, the temperature of the furnace surface at the time the refractory material is applied, second, the highest temperature to which the refractory coating will be subjected under operating conditions, and, third, the actual composition of the furnace surface lining refractory material.

In preparing the refractory material, the ingredients are finely pulverized and thoroughly mixed in proper propoti-ons. Sufficient water is then added to give the desired fluidity, giving it a specific gravity of about 2.2 to 2.6. The digestion of the slurry is assisted if it is heated.

The roof treating procedure may be effected by conventional apparatus of which an important item is the so-called spray gun or refractory gun, the long known principles of which are shown in the patent to Bodfish, No. 1,574,183 of February 23, 1926, this or the like being adapted to project the refractory material, under observation and control, so as methodically to cover all affected or exposed surfaces.

When the new refractory composition is brought into contact with the basic refractory surface of the furnace wall under operating temperature, the free iron oxide with the binding agents results in localized and limited fusing. Such fusion proceeds towards a state of a highly gelatinous liquid, but only until the ever-changing ratio of free metal oxides to the refractories within the refractory composition and those of which the wall is composed become such that solidification upon the roof and wall surface takes place. There is then a rearrangement of the compounds, and this setting or solidification is the end point of the reaction, the result being that a fused chemical or sintered bond is formed between the refractories of the wall and of the coating material.

As an example of the use of a commercial chrome ore as a basis for our refractory composition we may consider an ore the assay of which is as follows:

4 Sample A Percent Cr O 44.5 FeO 25.4 MgO 10.6 A1 0 14.6 Other 4.9

The formation of a spinel with 44.5 parts of C r O will bind within it 20.9 parts of the FeO, leaving 4.5 parts free FeO.

The excess of free iron is within the range of free iron oxide specified (1 to 6%) so that no more iron oxide need be added.

Then considering another:

SampleB Percent Cr O 45.4 FeO 15.1 MgO 13.6 Al O 13.8 Other 12.1

The formation of a chrome spinel with 45.4 parts of Cr O would require 21.4 parts of FeO. There is a lack therefore of 6.3 parts FeO to form the spinel and hence no free iron oxide.

A refractory composition using this source of chromite, therefore, would require 6.3 parts of FeO to complete the spinel plus 1 to 6 parts as free FeO. That is an addition of 7.3 to 12.3 parts FeO.

A chrome ore may contain natural variations in proportions and breaking down an analysis of the Sample A type into the proportions of each of the metal oxides, ignoring any incidental SiO the range may run somewhat as follows, in parts or percentages:

Cr O 25 to 50 FeO 10to 25 A1 0 8 to 26 MgO 5 to 20 The refractory material of the new composition, of whatever formula, should be finely pulverized, the following ranges of particle size being satisfactory as an example:

Percent To remain upon mesh screen 20 to 40 To remain upon 200 mesh screen 40 to 55 To pass 200 mesh screen 45 to 60 What is claimed is:

1. A coating composition adapted to be sprayed onto the hot basic refractory material forming the inner surface of a high temperature furnace while the furnace is in operation consisting of an aqueous slurry of sprayable consistency, said slurry containing a binding agent, a suspension agent and finely powdered refractory material consisting essentially of chrome ore, uncombined magnesia and uncombined iron oxide, the chrome ore constituting the major portion of the refractory material, the amount of uncombined magnesia in the refractory material being at least 10 parts by weight over and above any magnesia that may be present in the chrome ore, and the amount of uncombined iron oxide present in the refractory material being such as to provide an excess of from 1 to 6 parts by Weight over and above the amount required to form an iron-chrome spinel with the chrome oxide present in the chrome ore, whereby the uncombined iron oxide acts as a fluxing agent, under the high temperature of the furnace, for both the chrome ore and the uncombined magnesia present in said refractory material when sprayed on said surface, the major portion of the chrome ore of the refractory material being in the form of chromite, the particle size of the refractory material being such that 20% to 40% will remain on a 100 mesh screen, 40% to 55% will remain on a 200 mesh screen and 45% to 60% will pass through a 200 mesh screen.

2. A coating composition as recited in claim 1 wherein the slurry has a specific gravity of about 2.2 to about 2.6, the amount of suspension agent present constitutes from 1 to 5 parts by weight, the amount of binding agent present constitutes from 6 to 12 parts by weight, said binding agent having mixed therewith from 0.25 to 5 parts by Weight of a dispersing agent.

3. A coating composition adapted to be sprayed onto the hot basic refractory material forming an inner surface of a high temperature furnace While the furnace is in operation consisting of an aqueous slurry of sprayable consistency, said slurry containing a binding agent, a suspension agent and finely powdered refractory material consisting essentially of from 82 to 92 parts by weight of chrome ore, of at least 10 parts by weight of uncombined magnesia in excess of any magnesia that may be present in the chrome ore, and of an amount of uncombined iron oxide calculated to give an excess of from 1 to 6 parts by weight of the amount of iron oxide required to form an iron-chrome spinel with the chrome oxide present in the ore, whereby the uncombined iron oxide acts as a fluxing agent under the high temperature of the furnace, for both the chrome ore and the uncombined magnesia present in said refractory material when sprayed onto said surface, the major portion of the chrome ore of the refractory material being in the form of chromite, the particle size of the refractory material being such that 20% to 40% will remain on a 100 mesh screen, 40% to 55% will remain on a 200 mesh screen and 45% to 60% will pass through a 200 mesh screen.

4. A coating composition according to claim 3 Wherein the slurry has a specific gravity of from about 2.2 to about 2.6.

5. A coating composition adapted to be sprayed onto the hot surface of a hightemperature furnace of the basic refractory type while the furnace is in operation, said composition consisting of an aqueous slurry having a specific gravity of from about 2.2 to about 2.6 and consisting essentially of an intimate mixture of from 82 to 92 parts by weight of chrome ore, of from 1 to 6 parts by Weight of free iron oxide, and of from 10 to 40 parts by Weight of free magnesia, all of said ingredients being ground to a fine grain size for spraying, the chrome ore containing a major amount of chromite, and the free iron oxide being in excess of the iron oxide required to form an iron-chrome spinel and acting as a fluxing agent for both the chorme ore and the free magnesia when the slurry is sprayed onto the furnace wall, and said composition also comprising from 1 to 5 parts by weight of a suspension agent which aids in spraying the slurry upon the furnace surface as well as from 6 to 12 parts by weight of a binding agent which aids in preliminary bonding the solid portions of the composition to the furnace surface when the slurry is sprayed thereon, said binding agent containing from .25 to 5 parts by weight of a dispersing agent to modify said binding agent and aid it in preliminarily bonding the solid particles to the furnace surface, the particle size of the refractory material being such that 20% to 40% will remain on a 100 mesh screen, 40% to 55% will remain on a 200 mesh screen and 45% to 60% will pass through a 200 mesh screen.

6. The method of coating the hot basic refractory material forming the inner surface of a high temperature furnace While the furnace is in operation, said method comprising mixing a finely powdered refractory material with sufficient water to produce a slurry of sprayable consistency, said refractory material consisting essentially of chrome ore, uncombined magnesia and uncombined iron oxide, the chrome ore constituting the major portion of the refractory material, the amount of uncombined magnesia present in the refractory material being at least 10 parts by weight of said material over and above any magnesia that may be present in the chrome ore, and the amount of iron oxide present in the refractory material being such as to provide an excess of from 1 to 6 parts by weight over and above the amount required to form an iron-chrome spinel with the chrome oxide present in the chrome ore, the major portion of the chorme ore of the refractory material being in the form of chromite, and progressively spraying the slurry under high pressure upon the hot surface of the furnace while the furnace is in operation, whereby the heat of the furnace first causes the uncombined iron oxide present in the refractory material to fuse and act as a fluxing agent for both the chrome ore and uncombined magnesia to bind them to said surface and thereafter to set and harden the coating after it has been bonded to the furnace surface and form a solid integrated lining which will resist destruction by the furnace atmosphere and heat.

7. The method of coating the hot surface of a high temperature furnace while the furnace is in operation, the material of said surface being from the group consisting of magnesia, chrome magnesite and chrome, said method including the steps of mixing a finely powdered coating composition with sufficient Water to produce an aqueous slurry of sprayable consistency, said slurry consisting essentially of an intimate mixture of from 82 to 92 parts by weight of chrome ore, of from 10 to 40 parts by weight of free magnesia, and of from 1 to 6 parts by weight of free iron oxide over and above the amount required to form an iron-chrome spinel with the chrome oxide in the chrome ore, all of said ingredients being ground to a particle size such that 20% to 40% will remain on a mesh screen, 40% to 55% will remain on a 200 mesh screen and 45% to 60% will pass through a 200 mesh screen, the chrome ore containing a major amount of chromite, and the free iron oxide acting as a fluxing agent for both the chrome ore and the free magnesia when the slurry is sprayed onto the furnace wall, spraying the aqueous slurry under high pressure and in progressive fashion upon the hot surface of the furnace while the furnace is in operation to form a heat resisting coating thereon, and utilizing the furnace heat, first, to activate the free iron oxide fluxing agent and produce a fusing bond between the solid particles of the composition and the furnace surface as the coating is gradually built up by the spraying operation and, second, to set and harden the coating after it has been bonded to the furnace surface and convert it into a solid integrated lining which will resist destruction by the furnace atmosphere or furnace heat during periods of expansion, contraction and growth and use of the refractory furnace wall to which the coating is applied.

References Cited in the file of this patent UNITED STATES PATENTS 2,316,229 Berlek Apr. 13, 1943 2,394,304 Griffith Feb. 5, 1946 2,809,126 Murphyet al. Oct. 8, 1957 

1. A COATING COMPOSITION ADAPTED TO BE SPRAYED ONTO THE HOT BASIC REFRACTORY MATERIAL FORMING THE INNER SURFACE OF A HIGH TEMPERATURE FURNACE WHILE THE FURNACE IS IN OPERATION CONSISTING OF AN AQUEOUS SLURRY OF SPRAYABLE CONSISTENCY, AND SLURRY CONTAINING A BINDING AGENT, A SUSPENSION AGENT AND FINELY POWDERED REFRACTORY MATERIAL CONSISTING ESSENTIALLY OF CHROME ORE, UNCOMBINED MAGNESIA AND UNCOMBINED IRON OXIDE THE CHROME ORE CONSTITUTING THE MAJOR PORTION OF THE REFRACTORY MATERIAL, THE AMOUNT OF UNCOMBINED MAGNESIA IN THE REFRACTORY MATERIAL BEING AT LEAST 10 PARTS BY WEIGHT OVER AND ABOVE ANY MAGNESIA THAT MAY BE PRESENT IN THE CHROME ORE, AND THE AMOUNT OF UNCOMBINED ION OXIDE PRESENT IN THE REFRACTORY MATERIAL BEING SUCH AS TO PROVIDE AN EXCESS OF FROM 1 TO 6 PARTS BY WEIGHT OVER AND ABOVE THE AMOUNT REQUIRED TO FROM AN IRON-CHROME SPINEL WITH THE CHROME OXIDE PRESENT IN THE CHROME ORE, WHEREBY THE UNCOMBINED IRON OXIDE ACTS AS A FLUXING AGENT, UNDER THE HIGH TEMPERATURE OF THE FURNACE, FOR BOTH THE CHROME ORE AND THE UNCOMBINED MAGNESIA PRESENT IN SAID REFRACTORY MATERIAL WHEN SPRAYED ON SAID SURFACE, THE MAJOR PORTION OF THE CHROME ORE OF THE REFRACTORY MATERIAL BEING IN THE FORM OF CHROMITE, THE PARTICLE SIZE OF THE REFRACTORY MATERIAL BEING SUCH THAT 20% TO 40% WILL REMAIN ON A 100 MESH SCREEN, 40% TO 55% WILL REMAIN ON A 200 MESH SCREEN AND 45% TO 60% WILL PASS THROUGH A 200 MESH SCREEN. 